Molecular characterization of extracted dissolved organic matter from New Zealand coals identified by ultrahigh resolution mass spectrometry

Abstract

Abstract Knowing the composition, molecular size and structure of water-soluble organic compounds are the prerequisites for a better understanding of water-rock interactions and of the main controls on their release from natural organic matter in shale environments and coal seams over both geological and human timescales. In this study, extracted dissolved organic matter (DOM) from two lignites and three bituminous coals of New Zealand was analyzed by high resolution Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) combined with electrospray ionization (ESI) in negative ion mode after solid phase extraction (SPE). Compared to DOM in natural waters, the yields of extracted DOM of New Zealand coals were 10 to 100 times higher while the compounds showed an elevated aromaticity as indicated by lower H/C ratios. Elevated abundances of benzenepolycarboxylic acids (BPCAs) and naphthalenepolycarboxylic acids (NPCAs) with different numbers of carboxyl and hydroxyl groups in the FT mass spectra of all five samples have been confirmed by GC-MS. Oxygen containing compounds are the dominant elemental class in the extracted DOM of bituminous coals and lignites. However, the relative intensity distributions of individual oxygen classes as well as of the double bond equivalents (DBE) in the extracted DOM of the bituminous coals differ from those in the lignites. The low abundance of O9–16 in the bituminous coals might reflect the loss of oxygen via eliminations of carboxyl and carbonyl groups with increasing maturation. Furthermore, the extracted DOM of bituminous coals contains higher percentages of aromatic and condensed aromatic compounds than the extracted DOM of the lignites. A modified aromaticity index (AIcoal) was established to evaluate aromaticity of the extracted DOM based on sample-specific calculated fraction of oxygen numbers bound in π-bonds instead of an assumed average of 50%.